Switching power supplies achieve DC to DC voltage supply regulation to provide an output voltage from an input voltage. Goals for switching power supplies include high power density (such as watts per unit area or watts per unit volume) and reduced cost. Switching power supplies such as buck converters often apply two different voltages (such as ground and the input voltage) to a switching node. By using an inductor and capacitor filter coupled between the switching node and an output terminal supplying the output voltage, a DC output voltage is provided at a given level. Using pulse width modulation (PWM) to control the on time and off time for a high side switch coupled between the input voltage and the switching node, and also to control a low side switch coupled between the switching node and a ground potential, the duty cycle for the high side switch can determine the output voltage. For a switching converter in a buck topology the duty cycle is proportional to the ratio Vout/Vin. The high side and low side switches can be implemented with FET transistors that are sized to handle the current required by the output. An output inductor is series coupled between the switching node and the output voltage. In a buck converter, the switching node varies between the input voltage Vin and ground, so the output inductor must be large enough to maintain reasonable current ripple. In buck converters the output inductor is often one of the largest devices in terms of board area and volume. The output inductor is often the tallest component, which increases volume of the buck converter. The power transistors also have to be sized to handle the current and voltage variations expected.
A three-level switching power supply generates a third voltage that can be coupled to the switching node. The third voltage is typically one-half of the input voltage. In a three-level switching power supply the switching node alternates between the input voltage and one-half of the input voltage, or between one-half of the input voltage and ground. Thus for a three-level converter the magnitude of voltage transitions at the switching node is approximately one-half the magnitude of the voltage transitions that occur in a traditional switching power supply. These reduced voltage transitions at the switching node reduce voltage stress on the power supply circuit elements, potentially enabling three-level switching power supplies to be implemented with smaller and lower cost components such as smaller power transistors and a smaller output inductor. Output inductors are commonly employed in switching power supplies and are often the largest component with respect to circuit board area and spatial volume of a switching power supply. The reduced voltage transitions at the switching node results in reduced current ripple through the output inductor. For a switching duty cycle of 50% between the high and low voltages at the switching node, the current ripple may be reduced by approximately 75% for a three-level converter when compared to a switching power supply implemented without three levels. The use of the three level switching converter significantly reduces current ripple and also enables the employment of physically smaller inductors, thus increasing the power density.